In International Patent Application No. PCT/GB 91/02112 published as WO 92/09886 there is described a system for monitoring debris in a fluid. The system comprises at least one electrostatic sensor for producing a signal representing electrostatic charge associated with the fluid moving past the sensor and with debris and/or impurities carried by the fluid. The signal from the sensor is conditioned by a signal conditioner and the conditioned signal is processed by a signal processor together with another signal representing at least the charge associated with the moving fluid in order to produce a signal representing the electrostatic charge associated with the debris and/or impurities. In practice two electrostatic sensors are used at spaced apart locations and the signals therefrom are processed, i.e. correlated, in order to identify the existence of debris in the moving fluid. The system is extremely useful for detecting wear in machines because machine wear causes charge carrying particles to be generated which can be detected by the system.